This invention pertains to an aluminum electrolytic capacitor utilizing an electrolyte containing a phosphate salt, the cation of which is compatible with the cation of the electrolyte solute. More specifically, the invention pertains to the use of lower alkyl- and heterocyclic-substituted ammonium dihydrogen phosphates in electrolytes utilizing other than ammonium salts as solutes.
The role of added phosphate on the working electrolytes of aluminum electrolytic capacitors is poorly understood and has been confusing. One rule of thumb had it that phosphate additions were desirable in low voltage electrolytes but undesirable and voltage-limiting in the working electrolytes of high voltage (greater than 200V) electrolytic capacitors. Nevertheless, it is a common, well-established practice in the electrolytic capacitor art to add small amounts (0.1-0.2%) of ammonium dihydrogen phosphate to all glycol borate electrolytes, whether designed for either high (above 200V) or low (below 200V) voltage applications. The phosphate is added to ensure the continued passivation of the cathode foil and protect it against hydration. Hydration is accompanied by copious hydrogen gas evolution, and the disruptive effect of this gas evolution is a frequent cause of capacitor failure.
The service requirements for electrolytic capacitors have been continuously increasing in severity, and the level of protection afforded by 0.1-0.2% ammonium dihydrogen phosphate is proving to be inadequate. Moreover, there is a growing body of evidence that indicates that the presence of a phosphate in the electrolyte also helps to stabilize the oxide film on the anode. The stabilization manifests itself in low leakage current levels, but the mechanism by which this is achieved is not presently understood. For this function, as well, higher phosphate concentrations are desirable.
Experience indicates that increasing the ammonium dihydrogen phosphate concentration to 0.5% is definitely beneficial, and it may be desirable to increase the concentration to 1.0% or even higher. However, at 25.degree. C. the limit of solubility of ammonium dihydrogen phosphate in glycol is close to 1.0%, and, in the typical glycol-based electrolyte with its high solute content, the solubility limit is very much lower. In a glycol-borate electrolyte which contains 82.5% by wt. glycol, 17.0% by wt. ammonium pentaborate and 0.5% by wt. ammonium dihydrogen phosphate, it is possible to incorporate the 0.5% ammonium dihydrogen phosphate, but this amount of phosphate is extremely difficult to dissolve, and this concentration level is extremely difficult to maintain in production.
In the other electrolyte solvents, e.g., N,N'-dimethylformamide (DMF), N-methylpyrrolidinone (NMP) and butyrolactone (BLO), the problem becomes even more acute. In all these solvents, ammonium dihydrogen phosphate is almost completely insoluble. Yet, there is an equivalent need for incorporating a phosphate salt in electrolytes utilizing these solvents alone or mixed with each other or with glycol.
The problem is further exacerbated for high voltage electrolytes which incorporate nitro compounds as depolarizers. As we have shown in our copending patent application on such high voltage electrolytes, filed concurrently, the amount of depolarizer, e.g., p-nitrobenzoic acid, that can be added without seriously lowering the maximum anodization voltage, Vmax, is determined by the amount of phosphate that is added. For long-life, high-voltage capacitors free of gas formation, it may be desirable to add as much as 5% or more of the phosphate. There is no conceivable way to do this with ammonium dihydrogen phosphate even in glycol, and in other solvents or solvent mixtures the prospects are even more dismal.